The present invention relates to a rapid-setting .alpha.-cyanoacrylate based adhesive composition having good storage stability and, in particular, to an adhesive composition having a very fast setting time on wood and other substrates with a porous/acid surface.
.alpha.-cyanoacrylates of general formula ##STR2## are rapid-setting adhesives which are ideally suited for the adhesion of a large number of materials. The parts can be joined in an extremely short time of only a few seconds and the adhesive joints produced in this way have good mechanical strength characteristics. The adhesives are hardened by anionic polymerization initiated by even small traces of extremely weak basic-acting compounds (Lewis bases) such as, for example, water or methanol.
When using such .alpha.-cyanoacrylates adhesives on wood, leather and other porous/acid surfaces, there is the problem that the anionic polymerization of the .alpha.-cyanoacrylate is inhibited in spite of the high moisture content which should in fact aid the polymerization. As a result, the adhesive setting time is too long for practical use. In addition, such adhesives tend to penetrate the pores of the surfaces to be adhered to one another, so that in the case of porous material, the strength of the adhesive joint is impaired due to the protracted setting time.
However, because the distinct advantage of .alpha.-cyanoacrylate based adhesives is their short setting time, numerous measures have been attempted to reduce the setting times on wood and similar materials. One of these measures involves treating one of the wood surfaces to be joined with a primer and the other with an .alpha.-cyanoacrylate adhesive composition. Polymerization occurs on contacting the treated surfaces together. The required added operation of applying a primer is a serious disadvantage to such process. Another measure involves reducing the quantity of acid stabilizers present for satisfactory storage stability of the .alpha.-cyanoacrylate adhesive compositions. However, although this reduces the setting time, such adhesive compositions have an increased tendency to prematurely harden during storage.
Therefore, polymerization catalysts have been proposed which are suitable for use in single-component systems and which reduce the setting times of such .alpha.-cyanoacrylate adhesives without excessively impairing their storage stability. According to DE-OS No. 2,816,836 the setting time of .alpha.-cyanoacrylate adhesives is reduced by adding to the adhesive composition approximately 0.1 ppm or more of a macrocyclic polyether compound from the group of macrocyclic polyethers and their analogs. The most serious disadvantage of such accelerators relates to their synthesis which, even when the dilution principle is used, only supplies the desired product in low yields, because the tendency to produce intermolecular linkages which form chains is greater than that for intramolecular linkages which form macrocycles. Further, U.S. Pat. No. 4,170,585 suggests the addition of approximately 0.0001 to 20% by weight of a polyethylene glycol with a degree of polymerization of at least 3 or a non-ionic surfactant with a polyethyleneoxy content (degree of polymerization also at least 3) or mixtures thereof to .alpha.-cyanoacrylate based adhesive compositions. These compounds, however, have the disadvantage in that they have a great tendency to contain water and low molecular weight polyethylene glycol ether which are difficult to remove and spontaneously initiate polymerization when the compounds are added to .alpha.-cyanoacrylates.
The two specifications discussed above also describe in detail the disadvantages commonly associated with .alpha.-cyanoacrylate adhesives when used on wood, leather and other porous/acid surfaces as mentioned above.
It has now been surprisingly found that the above difficulties and disadvantages of the prior art polymerization catalysts can be obviated by the use of certain compounds described herein below as polymerization catalysts contained in .alpha.-cyanoacrylate-based adhesive compositions. These compounds can be easily prepared in high yield and purity, are free from polymerization-initiating substances, require only limited concentrations in the resulting adhesive compositions based on .alpha.-cyanoacrylate and lead to greatly reduced setting times on wood and other porous/acid surfaces. It has also been found that the susceptibility to water can be still further reduced by the addition of suitable compounds described herein below which result in improved storage stability of such adhesive compositions.